1. Field of the Invention
This invention relates to a method for performing a multi-layer welding operation automatically along predetermined paths of welding by reading out preinstructed points.
2. Description of the Prior Art
The control of the weaving path of a welding torch in response to variations in the welding current signal utilizes the phenomenon that the welding current is lowered by an increase in the distance from the contact tip to the base material when a predetermined voltage is applied to the arc welding wire through the contact tip. For example, when the weaving motions of a welding torch 101 to the left and right sides of the center line of a gap 100 are of the same width as shown in FIG. 1, the welding current takes a waveform as shown in FIG. 2, which consists of waves A-B-A and A-C-A of the same shape. However, if the weaving motion of the welding torch 3 is deviated to one side as shown in FIG. 3, the wave A-B-A becomes greater than the wave A-C-A, shown as an asymmetric waveform. In such a case, the average current values in the period A-C-A are compared and the position of the torch is corrected toward the side with a smaller average current. The torch position may be corrected by comparing the peak currents at points B and C instead of the average currents.
However, if the weaving motion is controlled by the above-described method in a multi-layer welding operation, the welding current of the second layer takes an asymmetric waveform as shown in FIG. 6 due to the influence of the existence of the first layer. In a case where the bead of the first layer is flat, the right half of the waveform is flattened, making it difficult to detect the peak value. Consequently, in the second and succeeding layers, it is impossible to discriminate the position of the torch correctly for the control of the weaving path simply by comparing the left and right waveforms of the welding current as in the welding operation of the first layer. Even if the control should employ different criteria in judging the position of the torch in the second and succeeding layers, it lacks versatility since the welding current takes different waveforms in the second, third and fourth layers and depends upon the welding condition.